A vehicle may operate over a wide range of environmental conditions. Some environmental conditions, such as altitude, may affect operation of some vehicle systems. For example, it may be more difficult for an engine to produce vacuum for the vehicle's vacuum system as the vehicle travels from a lower altitude where barometric pressure is higher to a higher altitude where barometric pressure is lower. Since barometric pressure is lower at higher altitudes, the engine may operate with substantially the same intake manifold pressure at equal torque but since atmospheric pressure is lower, intake manifold vacuum is reduced. Consequently, the engine may not be able to provide as deep of vacuum at altitude as compared to when the engine is operated at sea level. Therefore, the vehicle's vacuum system may not be able to provide as much vacuum assistance as is desired.
The inventors herein have recognized the above-mentioned issues and have developed a method for operating an engine, comprising: combusting an amount of a first fuel at an engine speed and load in absence of a vacuum request, the first fuel introduced to the engine in a gaseous state; and reducing the amount of the first fuel and increasing an amount of a second fuel introduced to the engine at the engine speed and load in response to presence of the vacuum request.
By reducing the amount of gaseous fuel supplied to an engine and substituting liquid fuel in place of the removed gaseous fuel, it may be possible to provide the technical result of increasing vacuum production by an engine. In particular, injection of liquid fuel rather than of gaseous fuel may allow an engine to operate with a deeper intake manifold vacuum while providing a same level of engine torque. This may be the case since liquid fuel displaces less volume in the engine's intake manifold, thereby allowing the engine to operate with a reduced MAP (e.g., deeper intake manifold vacuum). In some examples, the gaseous fuel may be a pressurized fuel such as compressed natural gas (CNG) or propane. In other examples, the gaseous fuel may be fuel vapors from gasoline that are introduced from the engine's crankcase or from a fuel vapor storage canister to the engine's intake system.
The present description may provide several advantages. For example, the approach may improve an engine's capacity to produce vacuum. Further, the approach may allow the vehicle to operate at higher altitudes while at the same time providing sufficient vacuum to operate the vehicle's vacuum actuators. Additionally, the approach may allow an engine to produce sufficient vacuum to enable further vacuum creation via an ejector.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.